Hydrogenation of β-Keto Sulfones to β-Hydroxy Sulfones with Alkyl Aluminum Compounds: Structure of Intermediate Hydroalumination Products

β-Hydroxy sulfones are important in organic synthesis. The simplest method of β-hydroxy sulfones synthesis is the hydrogenation of β-keto sulfones. Herein, we report the reducing properties of alkyl aluminum compounds R3Al (R = Et, i-Bu, n-Bu, t-Bu and n-Hex); i-Bu2AlH; Et2AlCl and EtAlCl2 in the hydrogenation of β-keto sulfones. The compounds i-Bu2AlH, i-Bu3Al and Et3Al are the at best reducing agents of β-keto sulfones to β-hydroxy sulfones. In reactions of β-keto sulfones with aluminum trialkyls, hydroalumination products with β-hydroxy sulfone ligands [R2AlOC(C6H5)CH2S(O)2(p-R1C6H4]n [where n = 1,2; 2aa: R = i-Bu, R1 = CH3; 2ab: R = i-Bu, R1 = Cl; 2ba: R = Et, R1 = CH3; 2bb: R = Et, R1 = Cl] and {[Et2AlOC(C6H5)CH2S(O)2(p-ClC6H4]∙Et3Al}n 3bb were obtained. These complexes in the solid state have a dimeric structure, while in solutions, they appear as equilibrium monomer–dimer mixtures. The hydrolysis of both the isolated 2aa, 2ab, 2ba, 2bb and 3bb and the postreaction mixtures quantitatively leads to pure racemic β-hydroxy sulfones. Hydroalumination reaction of β-keto sulfones with alkyl aluminum compounds and subsequent hydrolysis of the complexes is a simple and very efficient method of β-hydroxy sulfones synthesis.

In the solution of β-keto sulfones, a tautomeric equilibrium takes place that is, however, almost completely shifted towards the ketone form (Scheme 1).
In the solution of β-keto sulfones, a tautomeric equilibrium takes place that is, however, almost completely shifted towards the ketone form (Scheme 1). Scheme 1. An equilibrium of β-keto sulfone tautomers.
Recently, we have found that the reaction between β-keto sulfones and t-Bu2AlH leads to the formation of aluminum complexes with β-hydroxy sulfone ligands, which indicates the reduction of β-keto sulfone to β-hydroxy sulfone by the alkyl aluminum compound [26]. The results of these studies inspired the development of a method for the β-hydroxy sulfones synthesis that uses aluminum alkyls bearing hydrogen atoms in the β-position of the alkyl substituents as β-keto sulfone-reducing agents.
In this paper, a β-keto sulfone reduction by various alkyl aluminum compounds, followed by the hydrolysis of the obtained aluminum complexes to β-hydroxy sulfones, is presented. Despite many methods that have been previously developed for the synthesis of chiral β-hydroxy sulfones, simple and efficient methods for the synthesis of racemic derivatives are still missing. We found that the efficiency of the reduction of β-keto sulfones to β-hydroxy sulfones depends mostly on the type of aluminum compounds, while the structure of β-keto sulfones affects the reduction process and the efficiency of β-hydroxy sulfone production to a lesser extent. Reactions of β-keto sulfones with i-Bu3Al and Et3Al, followed by the hydrolysis of postreaction mixtures, appear as a simple, efficient and cheap method of synthesizing β-hydroxy sulfones from starting β-keto sulfones. During the reaction of β-keto sulfones with aluminum alkyl compounds, complexes of aluminum alkyls with β-hydroxy sulfones as hydroalumination products are formed. The crystalline complexes were isolated and characterized by X-ray.

Hydroalumination Reaction of β-Keto Sulfones
β-Keto sulfones 1a-1e were subjected to the reaction with alkyl aluminum compounds (i-Bu3Al, i-Bu2AlH, Et3Al, n-Bu3Al, n-Hex3Al, Et2AlCl and EtAlCl2), providing postreaction mixtures of β-keto sulfone hydroalumination products and the appropriate alkyl aluminum complex supported by β-keto sulfones. The compositions of the mixtures depended on the type of alkyl aluminum compounds and their reducing ability, as well as the structure of β-keto sulfones or the reaction conditions. The five hydroalumination products 2aa, 2ab, 2ba, 2bb and 3bb were isolated as crystalline solids, and their structures were examined in the solid state (Scheme 2). Moreover, all postreaction mixtures were subjected to hydrolysis in order to determine the degree of conversion of β-keto sulfones to β-hydroxy sulfones. Recently, we have found that the reaction between β-keto sulfones and t-Bu 2 AlH leads to the formation of aluminum complexes with β-hydroxy sulfone ligands, which indicates the reduction of β-keto sulfone to β-hydroxy sulfone by the alkyl aluminum compound [26]. The results of these studies inspired the development of a method for the β-hydroxy sulfones synthesis that uses aluminum alkyls bearing hydrogen atoms in the β-position of the alkyl substituents as β-keto sulfone-reducing agents.
In this paper, a β-keto sulfone reduction by various alkyl aluminum compounds, followed by the hydrolysis of the obtained aluminum complexes to β-hydroxy sulfones, is presented. Despite many methods that have been previously developed for the synthesis of chiral β-hydroxy sulfones, simple and efficient methods for the synthesis of racemic derivatives are still missing. We found that the efficiency of the reduction of β-keto sulfones to β-hydroxy sulfones depends mostly on the type of aluminum compounds, while the structure of β-keto sulfones affects the reduction process and the efficiency of β-hydroxy sulfone production to a lesser extent. Reactions of β-keto sulfones with i-Bu 3 Al and Et 3 Al, followed by the hydrolysis of postreaction mixtures, appear as a simple, efficient and cheap method of synthesizing β-hydroxy sulfones from starting β-keto sulfones. During the reaction of β-keto sulfones with aluminum alkyl compounds, complexes of aluminum alkyls with β-hydroxy sulfones as hydroalumination products are formed. The crystalline complexes were isolated and characterized by X-ray.

Hydroalumination Reaction of β-Keto Sulfones
β-Keto sulfones 1a-1e were subjected to the reaction with alkyl aluminum compounds (i-Bu 3 Al, i-Bu 2 AlH, Et 3 Al, n-Bu 3 Al, n-Hex 3 Al, Et 2 AlCl and EtAlCl 2 ), providing postreaction mixtures of β-keto sulfone hydroalumination products and the appropriate alkyl aluminum complex supported by β-keto sulfones. The compositions of the mixtures depended on the type of alkyl aluminum compounds and their reducing ability, as well as the structure of β-keto sulfones or the reaction conditions. The five hydroalumination products 2aa, 2ab, 2ba, 2bb and 3bb were isolated as crystalline solids, and their structures were examined in the solid state (Scheme 2). Moreover, all postreaction mixtures were subjected to hydrolysis in order to determine the degree of conversion of β-keto sulfones to β-hydroxy sulfones.
The molecular structures of compounds 2aa, 2ab, 2ba, 2bb and 3bb were determined by X-ray diffraction study and are shown in Figures 1-5. Data collection and structure analyses are listed in Tables S1 and S2 (see Supplementary Materials). In the solid state, all of the described compounds were presented as centrosymmetric dimers. They consisted of central four-membered Al2O2 rings formed by two monoanionic β-hydroxy sulfonic ligands and two alkylaluminium moieties with four-coordinate aluminum centrum. Additionally, in the 3bb molecule, there were two Et3Al molecules coordinated to the oxygen atoms in the SO2 groups. The sum of the angles around the O(3) atoms was 354.9° for compound 2aa and 354.7° for compound 2ab, which indicated slight stress in the central part of the molecule. Similarly, the sums of the angles around the oxygen atoms of the Al2O2 rings in compounds 2ba, 2bb and 3bb were 354.8, 354.7 and 355.6°, respectively. Scheme 2. Synthesis of β-hydroxy sulfones 4a-4e by hydroalumination of β-keto sulfones and hydrolysis of the compounds 2aa, 2ab, 2ba, 2bb and 3bb or hydrolysis of postreaction mixtures of the reactions of β-keto sulfones 1a-1e with R 3 Al (where R = i-Bu, Et) or R 2 AlH (where R = i-Bu).
The molecular structures of compounds 2aa, 2ab, 2ba, 2bb and 3bb were determined by X-ray diffraction study and are shown in Figures 1-5. Data collection and structure analyses are listed in Tables S1 and S2 (see Supplementary Materials). In the solid state, all of the described compounds were presented as centrosymmetric dimers. They consisted of central four-membered Al 2 O 2 rings formed by two monoanionic β-hydroxy sulfonic ligands and two alkylaluminium moieties with four-coordinate aluminum centrum. Additionally, in the 3bb molecule, there were two Et 3 (1),  In compound 3bb, due to the presence of Et 3 Al molecules coordinated to the oxygen atoms from the SO 2 groups, there was an additional triplet of (CH 3 CH 2 ) 3 Al protons and a quartet of (CH 3 CH 2 ) 3 Al protons (at 0.92 and −0.29 ppm, respectively) in the 1 H NMR spectrum ( Figure S13). In addition, there were four triplets at 1.03, 0.84, 0.74 and 0.63 ppm of CH 3 CH 2 Al protons; three quartets at −0.03, −0.46, −0.64 ppm and one quartet at −0.29 ppm overlapping the signal of the (CH 3 CH 2 ) 3 Al protons. In compound 3bb, due to the presence of Et3Al molecules coordinated to the oxy atoms from the SO2 groups, there was an additional triplet of (CH3CH2)3Al protons a quartet of (CH3CH2)3Al protons (at 0.92 and −0.29 ppm, respectively) in the 1 H N spectrum ( Figure S13). In addition, there were four triplets at 1.03, 0.84, 0.74 and The complex nature of the NMR spectra of 2aa, 2ab, 2ba, 2bb and 3bb complexes could be explained by the monomer-dimer equilibria in the solutions (Scheme 3). To confirm this, the molecular weight of the dissolved compounds was determined by the cryometric method. In the solid state, the compounds had the structures of dimeric (R*,S*) diastereomers, as shown by X-ray measurements (Figures 1-5). After dissolving the compounds, Al 2 O 2 rings in dimeric structures were easily dissociated to form monomeric structures stabilized by the formation of Al-O coordination bonds between the oxygen atoms of the SO 2 group and aluminum atoms. The association degrees calculated from the values of molecular weights ranged from 1.22 (for 3bb) to 1.50 (for 2ab), which means that, in solutions of compounds 3bb and 2ab, there were 22 and 49 mol% of the dimeric structure, respectively. Taking into account the results of NMR studies and molecular weight measurements, it can be concluded that hydroalumination products of β-keto sulfones exist as an equilibrium mixture of monomers-dimers in solutions (Scheme 3).
Since the tautomeric equilibrium in the β-keto sulfones solutions was almost completely shifted towards the ketone form, only this form was taken into account in the hydroalumination mechanism suggested. When i-Bu 2 AlH was used, the mechanism was based on the assumption of a charge distribution between the carbonyl C=O and Al-H groups, allowing the formation of an intermediate state. The oxygen atom in the C=O group with a partially negative charge interacted with a partially positive aluminum, and the partially negative charged hydrogen atom Al-H was transferred to the C=O carbon atom simultaneously (Scheme 4). We have recently proposed a similar mechanism for the hydroalumination of β-keto sulfones with t-Bu 2 AlH [26].
oxygen atoms of the SO2 group and aluminum atoms. The association degrees calculated from the values of molecular weights ranged from 1.22 (for 3bb) to 1.50 (for 2ab), which means that, in solutions of compounds 3bb and 2ab, there were 22 and 49 mol% of the dimeric structure, respectively. Taking into account the results of NMR studies and molecular weight measurements, it can be concluded that hydroalumination products of β-keto sulfones exist as an equilibrium mixture of monomers-dimers in solutions (Scheme 3).

Scheme 3. Equilibrium monomer-dimer mixtures of the hydroalumination products.
Since the tautomeric equilibrium in the β-keto sulfones solutions was almost completely shifted towards the ketone form, only this form was taken into account in the hydroalumination mechanism suggested. When i-Bu2AlH was used, the mechanism was based on the assumption of a charge distribution between the carbonyl C=O and Al-H groups, allowing the formation of an intermediate state. The oxygen atom in the C=O group with a partially negative charge interacted with a partially positive aluminum, and the partially negative charged hydrogen atom Al-H was transferred to the C=O carbon atom simultaneously (Scheme 4). We have recently proposed a similar mechanism for the hydroalumination of β-keto sulfones with t-Bu2AlH [26]. In the reactions of β-keto sulfones with i-Bu3Al an Et3Al, β-hydrogen from the i-Bu or Et group bonded to the partially positive C=O carbon, and the aluminum atom interacted with the negative oxygen atom C=O. An intermediate state involving six atoms, AlCCHCO, was formed. In the next step, the alkene molecule was removed, and the aluminum complex of β-hydroxy sulfone was formed (Scheme 4). The similar mechanism was previously published by Ashby for a ketone reduction reaction with i-Bu3Al [27].

Hydrogenation of β-Keto Sulfones to β-Hydroxy Sulfones
Reaction mixtures of β-keto sulfones with aluminum compounds were hydrolyzed to decompose the complexes. The obtained products were characterized by NMR spectroscopy to determine the molar ratio of β-hydroxy sulfone to β-keto sulfone on the basis of an integration of SO2CH proton signals in β-hydroxy sulfone and in β-keto sulfone. The yield of β-hydroxy sulfones (Table 1) illustrated an efficiency of the β-keto sulfone hydrogenation process. We determined the effect of the structure of β-keto sulfones, the type of aluminum compound and the reaction conditions on the efficiency of the hydrogenation of β-keto sulfones to β-hydroxy sulfones. We found that the hydrogenation reaction depended primarily on the nature of the aluminum alkyl compound. The most active reagent was i-Bu3Al, which reduced quantitatively all β-keto sulfones regardless of their structure. Et3Al was a good reducer for β-keto sulfones 1a,b and 1d,e, with elec-Scheme 4. The proposed mechanism for β-keto sulfone hydroalumination with i-Bu 2 AlH, i-Bu 3 Al and Et 3 Al. In the reactions of β-keto sulfones with i-Bu 3 Al an Et 3 Al, β-hydrogen from the i-Bu or Et group bonded to the partially positive C=O carbon, and the aluminum atom interacted with the negative oxygen atom C=O. An intermediate state involving six atoms, AlCCHCO, was formed. In the next step, the alkene molecule was removed, and the aluminum complex of β-hydroxy sulfone was formed (Scheme 4). The similar mechanism was previously published by Ashby for a ketone reduction reaction with i-Bu 3 Al [27].

Hydrogenation of β-Keto Sulfones to β-Hydroxy Sulfones
Reaction mixtures of β-keto sulfones with aluminum compounds were hydrolyzed to decompose the complexes. The obtained products were characterized by NMR spectroscopy to determine the molar ratio of β-hydroxy sulfone to β-keto sulfone on the basis of an integration of SO 2 CH proton signals in β-hydroxy sulfone and in β-keto sulfone. The yield of β-hydroxy sulfones (Table 1) illustrated an efficiency of the β-keto sulfone hydrogenation process. We determined the effect of the structure of β-keto sulfones, the type of aluminum compound and the reaction conditions on the efficiency of the hydrogenation of β-keto sulfones to β-hydroxy sulfones. We found that the hydrogenation reaction depended primarily on the nature of the aluminum alkyl compound. The most active reagent was i-Bu 3 Al, which reduced quantitatively all β-keto sulfones regardless of their structure. Et 3 Al was a good reducer for β-keto sulfones 1a,b and 1d,e, with electronwithdrawing substituents in the β-position, while the hydrogenation of β-keto sulfone 1c with an electron-donating methyl group was 75% efficient. Using an excess of Et 3 Al slightly increased the yield of β-hydroxy sulfone 4c to 82% (Table 1, run 3). The activity of n-Bu 3 Al, n-Hex 3 Al and t-Bu 3 Al in the hydrogenation of β-keto sulfones was weaker compared to the activity of i-Bu 3 Al and Et 3 Al. However, using an excess of n-Hex 3 Al and t-Bu 3 Al to reduce the β-keto sulfones 1a and 1b resulted in a significant increase in yield from 55 to 100% and from 8 to 92%, respectively ( Table 1, runs 1 and 2). The presence of chloride substituents in alkyl aluminum compounds significantly reduced the activity of these compounds in the hydrogenation of β-hydroxy sulfones. In the presence of an equimolar amount of Et 2 AlCl only 17% of the beta keto sulfone, 1b was reduced. For a 1:2 molar ratio of Et 2 AlCl:1b, β-hydroxy sulfone 4b was obtained with a yield of 25% (Table 1, run 2). EtAlCl 2 was inactive in the hydrogenation of β-keto sulfones ( Table 1, run 1).  1c
The nature of the starting β-keto sulfones had a less significant effect on their ability to be hydrogenated with alkyl aluminum compounds. The presence of electron-withdrawing groups on the C=O carbon atom, such as the phenyl substituent in compounds 1a,b and 1d,e, caused an increase in the partial positive charge on the C=O carbon atom, which favored the reduction of β-keto sulfones, as shown in the Scheme 4.
Earlier studies on ketone hydrogenation showed that the presence of a Lewis base (e.g., diethyl ether, THF) inactivates the reducing properties of aluminum alkyls [31]. That was why we used methylene dichloride, n-pentane and n-hexane as solvents; however, methylene dichloride proved to be the best due to the good solubility of the compounds.
The reaction of aluminum alkyls with β-keto sulfones and subsequent hydrolysis of postreaction mixtures was a simple method of β-keto sulfones hydrogenation. However, this method was suitable when the β-keto sulfone was completely hydroaluminated by an alkyl aluminum compound. On the other hand, in the presence of less active aluminum alkyls, only a part of the β-keto sulfone could be hydroaluminated. Then, in the postreaction mixture, there were alkyl aluminum complexes with β-hydroxy sulfone and β-keto sulfone ligands, which, after hydrolysis, yielded a mixture of β-hydroxy sulfone  100:0 100:0 100:0 4e a Molar ratio of β-keto sulfone:alkyl aluminum reagent. b Molar ratio of β-hydroxy sulfone:β-keto sulfone in the reaction products based on 1 H NMR spectra. c The isolated hydroalumination reaction product of β-keto sulfone with aluminum compounds 2aa, 2ab, 2ba, 2bb and 3bb were subjected to hydrolysis.
The nature of the starting β-keto sulfones had a less significant effect on their ability to be hydrogenated with alkyl aluminum compounds. The presence of electron-withdrawing groups on the C=O carbon atom, such as the phenyl substituent in compounds 1a,b and 1d,e, caused an increase in the partial positive charge on the C=O carbon atom, which favored the reduction of β-keto sulfones, as shown in the Scheme 4.
Earlier studies on ketone hydrogenation showed that the presence of a Lewis base (e.g., diethyl ether, THF) inactivates the reducing properties of aluminum alkyls [31]. That was why we used methylene dichloride, n-pentane and n-hexane as solvents; however, methylene dichloride proved to be the best due to the good solubility of the compounds.
The reaction of aluminum alkyls with β-keto sulfones and subsequent hydrolysis of postreaction mixtures was a simple method of β-keto sulfones hydrogenation. However, this method was suitable when the β-keto sulfone was completely hydroaluminated by an alkyl aluminum compound. On the other hand, in the presence of less active aluminum alkyls, only a part of the β-keto sulfone could be hydroaluminated. Then, in the postreaction mixture, there were alkyl aluminum complexes with β-hydroxy sulfone and β-keto sulfone ligands, which, after hydrolysis, yielded a mixture of β-hydroxy sulfone 4e a Molar ratio of β-keto sulfone:alkyl aluminum reagent. b Molar ratio of β-hydroxy sulfone:β-keto sulfone in the reaction products based on 1 H NMR spectra. c The isolated hydroalumination reaction product of β-keto sulfone with aluminum compounds 2aa, 2ab, 2ba, 2bb and 3bb were subjected to hydrolysis.
The nature of the starting β-keto sulfones had a less significant effect on their ability to be hydrogenated with alkyl aluminum compounds. The presence of electron-withdrawing groups on the C=O carbon atom, such as the phenyl substituent in compounds 1a,b and 1d,e, caused an increase in the partial positive charge on the C=O carbon atom, which favored the reduction of β-keto sulfones, as shown in the Scheme 4.
Earlier studies on ketone hydrogenation showed that the presence of a Lewis base (e.g., diethyl ether, THF) inactivates the reducing properties of aluminum alkyls [31]. That was why we used methylene dichloride, n-pentane and n-hexane as solvents; however, methylene dichloride proved to be the best due to the good solubility of the compounds.
The reaction of aluminum alkyls with β-keto sulfones and subsequent hydrolysis of postreaction mixtures was a simple method of β-keto sulfones hydrogenation. However, this method was suitable when the β-keto sulfone was completely hydroaluminated by an alkyl aluminum compound. On the other hand, in the presence of less active aluminum alkyls, only a part of the β-keto sulfone could be hydroaluminated. Then, in the postreac-tion mixture, there were alkyl aluminum complexes with β-hydroxy sulfone and β-keto sulfone ligands, which, after hydrolysis, yielded a mixture of β-hydroxy sulfone and β-keto sulfone. In order to avoid a difficult separation of β-hydroxy sulfone from this mixture, the alkyl aluminum complex with β-hydroxy sulfone ligands should be crystallized from the reaction mixture and then hydrolyzed to pure β-hydroxy sulfone. Complexes with β-keto sulfone ligands were thick liquids, which facilitated the separation of solid complexes with β-hydroxy sulfone ligands.

General Remarks
All manipulations were carried out using standard Schlenk techniques under an inert gas atmosphere. Methylene dichloride was deacidified with basic Al 2 O 3 and distilled over P 2 O 5 under argon. 1 H and 13 C NMR spectra were obtained on a Varian Mercury-400 MHz spectrometer (Varian International AG, Switzerland). Chemical shifts were referenced to the residual proton signals of CDCl 3 (7.26 ppm). 13 C NMR spectra were acquired at 100.60 MHz (standard: chloroform 13 CDCl 3 , 77.20 ppm). NMR spectra can be found in the Supporting Information (Figures S1-S15). Tri-iso-butyl aluminum and di-isobutyl aluminum hydride were from Sigma-Aldrich Company (Poznań, Poland). β-Keto sulfones 1a-e were synthesized according to the literature data [44]. Hydrolysable alkyl groups bonded to Al atoms for products 2aa, 2ab, 2ba, 2bb and 3bb were determined by hydrolysis of the compound (0.2 to 0.3 g) using HNO 3 solution (10% concentrated, 5 cm 3 ) and measurement of the volume of gaseous alkane (C 4 H 10 or C 2 H 6 ). Subsequently, the sample was transformed into Al 2 O 3 by mineralization, and the obtained white solid was dissolved in a diluted water solution of HNO 3 . The content of aluminum was determined by the complexation of Al 3+ cations with versenate anions using an excess of the titrated solution of calcium disodium versenate. Then, the excess of calcium disodium versenate was titrated by FeCl 3 .

X-ray Crystallography
The X-ray measurements of compounds 2aa, 2ab, 2ba, 2bb and 3bb were performed at 100(2) K on a Bruker D8 Venture Photon100 diffractometer equipped with a TRIUMPH monochromator and a MoKα fine focus-sealed tube (λ = 0.71073 Å). The total frames were collected with the Bruker APEX2 program [45]. The temperature of the samples was 100 K. The frames were integrated with the Bruker SAINT software package [46] using a narrow frame algorithm. Data were corrected for absorption effects using the multi-scan method (SADABS) [47]. The structures were solved and refined using the SHELXTL software package [48,49]. The atomic scattering factors were taken from the International Tables [50]. All hydrogen atoms were placed in calculated positions and refined within the riding model. Detailed crystallographic data are listed in Tables S1 and S2.

Reactions of β-Keto Sulfones with Alkyl Aluminum Compounds-General Procedure
A solution of a suitable amount of alkyl aluminum compound in methylene dichloride was added to a solution of 2 mmol of β-keto sulfone in 10 cm 3 of methylene dichloride at 0-5 • C with stirring. After warming up to room temperature, the postreaction mixture was subjected to hydrolysis.

Preparation of Hydroalumination Products
Reactions of i-Bu 3 Al, i-Bu 2 AlH and Et 3 Al with β-Keto Sulfones A solution of i-Bu 2 AlH (0.284 g, 2 mmol) or i-Bu 3 Al (0.396 g, 2 mmol) in 10 cm 3 of methylene dichloride was added to a solution of β-keto sulfone (0.548 g, 2 mmol of 1a or 0.589 g, 2 mmol of 1b) in 10 cm 3 at 0-5 • C with stirring. A solution of Et 3 Al (0.228 g, 2 mmol) in 10 cm 3 of methylene dichloride was added to a solution of β-keto sulfone (0.548 g, 2 mmol of 1a or 0.589 g or 2 mmol of 1b) in 10 cm 3 at −76 • C with stirring. A solution of Et 3 Al (0.456 g, 2 mmol) in 20 cm 3 of methylene dichloride was added to a solution of β-keto sulfone 1b (0.589 g, 2 mmol) in 10 cm 3 at −76 • C with stirring. The mixtures were stirred for 1 h at this temperature and then allowed to warm to ambient temperature. The solvent was removed from the postreaction mixtures by distillation under vacuum. A thick liquid was obtained when the reagent was i-Bu 2 AlH, while white solids were obtained when the reagents were i-Bu 3 Al and Et 3 Al. White crystals of the complexes 2aa, 2ab, 2ba, 2bb and 3bb suitable for X-ray measurements were precipitated from n-C 6 H 14 /CH 2 Cl 2 solutions. Before measuring the molecular weight by the cryoscopic method in benzene and analysis, samples of compounds were placed under vacuum (10 −2 Torr) for 5 h to remove the solvent. Yield: i-Bu 3 Al reacted with β-keto sulfones 1a and 1b, yielding compounds 2aa and 2ab quantitatively (based on NMR spectra), while postreaction mixtures of i-Bu 2 AlH with β-keto sulfones 1a and 1b, besides 2aa and 2ab, consisted of side products.